Energy storage is important in many energy consumption applications. Such applications include conventional and renewable utility power generation, air-conditioning and heating of buildings, industrial process heating, and the like. Energy storage permits nuclear and coal-fired power plants to shift power generated during low demand hours to peak demand hours. Energy storage to meet grid demand is even more important for wind and solar power plants since the power output of these energy resources varies with time of day, and weather conditions.
Direct electric energy storage typically utilizes batteries. Although there are continuing efforts to improve battery technologies, the intrinsic high cost of batteries limits their application to small scale emergency power supplies. Hydroelectric and compressed air storage systems are two current solutions where electricity is first converted to potential energy of water or air by pumps and compressors, and then converted back to electricity when needed. However, these solutions typically require special terrain and/or geological conditions, i.e. terrain permitting the building of low and high attitude water reservoirs, and/or natural underground air-tight high pressure air-reservoirs. Such conditions are rarely available for exploitation by local power plants.
Thermal energy storage is intrinsically low cost, due to the availability of various low cost materials for this use. Since most power plants (more than 80%) generate electricity through thermal processes, thermal energy storage can be conveniently applied to utility power generation.
Thermal energy storage is also important for concentrated solar power (CSP) plants. The working principle of CSP is to use focusing mirrors (e.g., parabolic dish mirrors, parabolic trough mirrors, Fresnel mirrors, and other types of focusing mirrors) to focus the solar radiation on a thermal collector, where special coating(s) convert the light into thermal energy. The thermal energy heats up a heat transfer fluid (HTF) flowing through the thermal collector to a certain hot temperature. The hot heat transfer fluid is then used to generate high pressure high temperature steam via heat exchanger(s) to drive steam turbine(s) for electricity generation. CSP power systems thus typically replace fossil fuel or nuclear fuel boilers with a solar heater/boiler, while keeping the other portions of the conventional power plant essentially unchanged.
However, because of variations in the “availability” of solar radiation, caused by clouds, inclement weather, the day/night cycle, and the like, a thermal storage sub-system is desirable in CSP plants in order provide a more constant source of power and to qualify them as a base load power supplier. Accordingly, a low cost and highly efficient thermal storage solution is important for CSP plants to be deployed in large scale to replace fossil fuel power plants. For example, for a given parabolic trough CSP power plant, without a thermal storage sub-system, the annual operation coefficient (percentage of time the CSP generates power) will be about 20%, i.e., approximately 1760 operating hours per year. However, if a thermal storage sub-system is used in conjunction with the CSP, the operating coefficient can be increased to more than 60% or approximately 5260 operating hours per year.
In typical thermal storage applications, there are three key thermal media: a heat transfer fluid (HTF), a thermal storage medium, and a working medium. Heat transfer fluids (HTF's) transfer the heat from a heat generator or collector to thermally charge a thermal storage medium, or to directly heat a working medium through for example, a heat exchanger. In certain implementations, a thermal storage medium charges (stores heat) by receiving the heat from a heat transfer fluid (HTF) and then later discharges the heat back to a heat transfer fluid (e.g., during hours of low insolation) that then delivers the heat to a working medium through a heat exchanger. The working medium receives the heat from the heat exchanger and drives the heat engine.
Heat transfer fluids are typically either a gas or a liquid. There two types of liquid HTF in common use. One is heat conducting oil and the other is molten salt. Normally, the highest temperature heat conducting oil can sustain is about 400° C. Above this temperature heat conducting oils typically decompose. Molten salt, on the other hand, can sustain a temperature up to about 600° C. However, molten salts typically must be continuously kept at a temperature higher than about 220° C. in order to avoid solidification and subsequent damage to transport pipes and containers. This high temperature requirement normally results in higher system maintenance costs. For gas type HTFs, steam can be used, however, however, high temperature steam is costly and requires high pressures. In certain implementations, hot air can be used, but hot air has a very low heat capacity and to conduct the same amount of heat per unit time a very high flow velocity is required. Considerable power is consumed to maintain this high flow velocity which substantially reduces the overall efficiency of systems that use hot air as a heat transfer fluid (HTF)>
In many systems, the working medium is a liquid, such as water, which is pre-pressurized to a desired working pressure, and heated to the desired working temperature via a heat exchanger, and finally released under pressure to undergo a liquid to gas phase transition. High pressure vapor is typically further over heated to eliminate water droplets before reaching the entrance to a heat engine, and is expanded at the entrance of the heat engine. During this process, the potential energy of pressurized molecules transforms to kinetic energy due to the expansion. This kinetic energy provides the driving force of the heat engine to produce mechanical work and, finally, to generate electricity. Such systems typically require high temperature, high pressure working media. For example, a typical 1 MW steam turbine requires ˜2.4 MPa pressure at a temperature of about 355° C. to achieve the greatest efficiency. For a typical 100 MW steam turbine, the required steam pressure and temperature increases to about 10 to 12 MPa at a temperature of about 380° C. to 400° C. Large size steam turbines can usually achieve higher conversion efficiencies of thermal energy to electricity, but in order to achieve this, they also require steam at a higher pressure and temperature.
Two approaches to store thermal energy based on the types of heat absorbed in materials include methods that exploit the storage of sensible heat and methods that exploit the storage of latent heat.
Sensible heat storage mechanisms may be based on the specific heat capacity of the storage medium, where the charging and discharging of thermal energy to and from the storage medium can be realized by increasing or decreasing the temperature of the materials as illustrated by the formula: Q=MCP(T2−T1)=MCPΔT (Eq. 1), where Q is the sensible heat stored in the heat storage medium, M is the mass of the heat storage medium, C is the specific heat capacity of the heat storage medium, T1 and T2 are the starting and ending temperatures, respectively, and ΔT is the temperature difference. Sensible heat storage is the most common, simple, mature and widely used thermal storage method. It can be further classified into four different methods: 1) liquid phase, 2) solid phase, 3) liquid and solid mixed phase, and 4) pressurized vapor sensible heat storage.
Liquid phase sensible heat thermal storage devices normally use either direct or indirect heat exchange methods. For example, a CSP solar collecting field, such as a parabolic trough or linear Fresnel mirror system, normally uses a conducting oil (mineral oil or synthetic oil) as its HTF in conjunction with molten salt as a liquid phase sensible heat thermal storage material. Such liquid-phase sensible storage materials are most often used in so called “Active Thermal Energy Storage” systems, where storage materials circulate through heat exchangers and collectors. In such systems a heat exchanger is typically used to transfer thermal energy from conducting oil to molten salt to store the thermal energy. Therefore, this method is called indirect thermal storage. Currently, most distributed solar thermal collecting fields (such as parabolic dish, parabolic trough, and linear Fresnel CSP) use such methods, which are presently one of the only two commercialized mature thermal energy storage methods.
The other commercialized thermal energy storage method is so called direct thermal storage. Systems that provide direct thermal storage with sensible heat typically utilize two tanks, one for high temperature molten salt and the other one for low temperature molten salt. During thermal energy storage (charging), high temperature conducting oil (HTF) transfers heat via a heat exchanger to the low temperature molten salt which flows or is pumped from the low temperature container to the high temperature container. The resulting high temperature molten salt is then stored in the high temperature container. When solar energy is not available, the storage system discharges to provide heat for electricity generation. When discharging heat, the high temperature salt flows or is pumped through a heat exchanger to the low temperature container. The heat exchanger transfers heat to generate high temperature high pressure steam for electricity generation. This discharge process comes to an end when most of the high temperature molten salt flows out from the high temperature container.
There are several problems with this approach: First, it requires several high temperature specialty pumps that can pump high temperature and very corrosive molten salt between the two containers, the conducting oil-molten salt heat exchanger, and the molten salt-steam generation heat exchanger. Second, it requires a specialized heat exchanger due to the corrosive nature of molten salt. Finally, the construction cost is still quite high: for example, for large scale deployment, the construction cost of such storage devices can be approximately $40/kWh of heat.
In certain implementations, a molten salt two container storage systems can also be configured as a direct thermal energy storage sub-system for a CSP system. Typically, in such implementations, the molten salt acts both as HTF for the solar collecting field and as a liquid phase sensible heat thermal storage material, i.e., HTF and sensible heat thermal storage material become the same material. Because no extra heat exchanger is involved, this approach is sometimes called direct thermal energy storage. This approach avoids a heat exchanger, which reduces thermal energy loss during the process. It is suitable for parabolic trough systems and works at approximately the 400° C.-500° C. temperature range. The main shortcoming with this approach is that extra heating devices and energy are typically required to keep the molten salt temperature above 220° (which is common molten salt's melting point), in order to avoid damage to the transport piping system. For a distributed solar collecting field, this significantly increases the complexity, the cost for the transport pipe(s), both in their construction and in their maintenance and services, and reduces the overall efficiency of the system.
Tower CSP systems can use direct liquid phase sensible heat thermal energy storage systems. One example is the Solar Tres tower CSP power plant in Spain. Because the transport piping system is vertically installed in the CSP tower, the liquid molten salt is easily discharged from the pipes so that the solidification problem is not as severe as in the parabolic trough CSP system. In addition, since the working temperature of tower CSP systems is normally significantly higher than that of parabolic trough CSP systems, the sensible heat thermal storage approach is more suitable to the tower CSP than for the trough CSP. For typical liquid phase temperature ranges in such systems, a mixture of inorganic salts or a single phase compound is used. For example, the Solar Two tower CSP in Nevada (United States) used 60% sodium nitride and 40% potassium nitride as a single phase compound. The melting point of this mixture is 220° and the working range is approximately 300° C.-600°. The SEGS trough system built in California desert the 1990s used therminol VP-1, Hitech (a 53% KNO3+7% NaNO3+40% NaNO2 mixture) and Hitec XL (a 45% KNO3+48% Ca(NO3)2+7% NaNO3 mixture) as direct liquid sensible heat thermal energy storage materials.
Solid state sensible heat thermal energy storage uses low cost materials such as rock, concrete, sand, and the like as thermal storage media. Since the solid materials cannot be transported between containers for thermal energy transportation, a gas phase or liquid phase HTF is also used for heat exchange media between the storage medium and working medium. This type of system is also called a “Passive Thermal Energy Storage” system. In direct steam generation CSP systems, the thermal storage system normally uses solid state sensible heat thermal energy storage materials. The greatest advantage is the low cost for such thermal storage materials. However, such systems are only used in indirect thermal energy storage approaches.
Tamme from Germany Aero Space Center (DLR) studied and developed high temperature concrete and cast ceramic as solid state sensible heat thermal energy storage materials based on a study of sand-rock concrete and basalt concrete properties, where the frame for the high temperature concrete is ferric oxide, and the cement acts as filling material. A disadvantage of solid state sensible storage methods is that the heat exchange and working temperature decreases during discharge, since the temperature of the sensible heat storage materials decreases as thermal energy (content) decreases. Another problem of such systems is that the thermal conductivity and heat transfer efficiency is low. Also if direct generated steam is used for the HTF, as it currently is, this requires the transport piping systems to cross the entire solar collecting field and the thermal storage containers must be constructed to sustain high temperatures and high pressures. This dramatically increases the cost for such steam transport as well as the thermal storage container cost. On the other hand, to reduce such costs, the pressure of the directly generated steam can be lowered. However this decreases the working efficiency of the turbine(s) driven by this steam. As consequence, this approach has been researched for a long time without major breakthroughs.
Liquid-solid state combined sensible heat thermal energy storage approaches use solid state storage materials and heat transfer fluids (HTFs) that are compatible at high temperature so that the solid state material and the HTF can be combined together to increase the heat capacity for the combined thermal storage system. One advantage of using solid state materials in thermal storage is to significantly reduce the usage of HTF while keeping the total amount of thermal storage unchanged so that the thermal storage cost can be lowed (in general, solid state material costs are much lower than that of the HTF). In order to reduce the equipment investment cost for the two tank liquid phase molten salt thermal energy storage system, thermocline tank storage systems have been tested (e.g., 2.3 MWh system at Sandia National Laboratory). The thermocline tank storage system utilizes a thermocline layer formed due to natural temperature cline distribution based on the relationship between thermal storage material density and the temperature.
A thermocline layer is formed when there is a temperature difference between the top (high temperature portion) of the thermal storage tank and the bottom (the low temperature portion) of the thermal storage tank. The thermocline layer acts as an insulation layer so that the molten salt on the top can be kept at a higher temperature and the molten salt on the bottom can kept at a lower temperature. During a thermal charging (thermal storage) period, the thermocline layer moves in an upward direction. During a thermal energy release (discharge) period, the thermocline layer moves downwards. In this way, the output molten salt is kept at a constant temperature. However, when the thermocline layer reaches the top of the tank or reaches the bottom of the tank, the temperature of the molten salt changes dramatically. In order to maintain the temperature layer gradient, one needs to strictly control the input and output of molten salt, as well as to properly arrange the solid state filling material into a layered structure, paired with floating inlet and ring-shell heat exchanger devices. Although this approach may reduce the thermal storage cost by 35% as compared to the previously described liquid phase sensible heat thermal energy storage system, it still has similar shortcomings to those mentioned above.
The CSP power plant of Planta Solar 10 (PS10) at Seville Spain uses pressurized steam at 285° C. with 4 MPa pressure to store the thermal energy. PS10 uses a high pressure container to store the pressured high temperature water flowing directly from a heat source or collector through high pressure pipes. This thermal storage approach smoothes the solar radiation intensity fluctuations during the day and can provide one hour of steam to the turbine power generator. When the pressurized high temperature water is released from the storage vassal, it undergoes a liquid-gas phase transition as the pressure is slightly reduced. The resulting high pressure steam can be used directly to drive a steam turbine. Strictly speaking, the stored energy here is still provided as sensible heat from high pressure water, not latent heat which only exists upon liquid-gas phase transition outside the storage tank. This is an effective method to provide a balanced load for a steam turbine. However, due to the high cost of pressurized vessels, this approach is very difficult to deploy at a large scale.
A working medium typically absorbs most energy near the working temperature, i.e. the temperature at the entry of the heat engine. This is due to large latent heat absorbed at the liquid to gas phase transition or to the large heat capacity of the medium near its critical point the liquid turns into gas phase regardless of the pressure. As a consequence, a sensible heat storage medium has to provide all needed thermal energy at this temperature. In order to do this, sensible heat storage media need to be charged to a much higher temperature according to the Eq. 1 (above). This relationship can also be expressed by following equation:Q=MwLwphase(@Tphase)=MHTFCP-HTFΔT  (Eq. 1B)where Mw and Lwphase are the working medium flow mass and the phase change latent heat. According to Eq.1B, the thermal energy required for working medium at its phase change temperature (Tphase) is about a few hundred times higher than the heat capacity per degree of the sensible heat storage materials (Cwphase>100CP-HTF). The product of ΔT and the mass M (proportional to the flow rate) has to be on the order of hundreds in order to provide enough thermal energy to produce the phase change. This approach requires vast quantities of heat transfer fluid (HTF) and a high volume of HTF circulation which also consumes energy. This requirement presents many challenges to the sensible thermal storage systems. Such challenges include: 1) Heat loss in transfer pipes and storage container, as well as in collector if the heat is from solar energy will be very high due to thermal radiation and convention, and difficult to control to an acceptable level. 2) It requires the HTF also work at this higher temperature. Usually the acceptable working temperature of HTF limits the temperature T2 and in turn limits the phase change temperature of the working medium. A lower phase change temperature of the working medium will result a lower efficiency of the heat engine.
To match the large thermal energy demand near the working temperature of the working medium, a phase change material that has a transition temperature at the working temperature can be used as the thermal storage medium where the large latent heat absorbed or released at the phase transition matches the demand. Furthermore, to provide a storage medium that can store such large amount of heat, the HTF can also to be a phase change material, otherwise a very large flow rate (100 times larger than the flow rate of working medium) typically must be adopted for the HTF, or a very high working temperature is required for the HTF.
Latent heat storage mechanisms utilize the heat associate with a material's physical state change, such as liquid to gas, solid to liquid, solid crystalline phase to phase transition. Latent heat associated with the transition has much higher effective specific heat capacity within transition the phase transition than that of sensible heat storage materials.
Heat absorbed or released at the phase transition is described by: Q=MCp(eff) δT=MLPCM (Eq. 2), where M is the mass of the materials, Cp(eff) is the effective heat capacity within phase transition, δT is the temperature difference within the transition range, and LPCM is the latent heat of PCM at the transition. Latent heat storage matching the heat demand of a working medium near its working temperature, lowers the required working temperature of the HTF and heat collectors, and therefore heat loss; and improves the efficiency of the heat engine. However, liquid-gas phase change latent heat storage is difficult to use due to the extremely large volume change that accompanies the phase change. Water, for instance, expands 1600 times when it vaporizes at 0.1 MPa pressure (one atmosphere). Therefore, it has not proven economical to utilize latent heat for thermal energy storage with a liquid-gas phase transition, because a large container with very high pressure inside the storage container was thought to be required to accommodate the gas phase volume, resulting significantly reduced thermal energy storage density and difficult mechanical structure design for the thermal storage apparatus.
In Europe, thirteen countries proposed a design of PCM storage system, referred to as the DISTOR project. In this project, direct generated steam or (high pressure water) is used as HTF, and graphite and PCM micro-encapsulated compound storage materials are used. Other methods involve mixed PCMs have also been proposed. In such previous proposals, heat exchange between HTF/WF and storage materials utilize a shell-piping heat exchanger, where HTF/WF flow in piping and PCMs surround piping inside the tank with solid filling materials to improve the thermal contact.
Although there are many studies on PCM thermal storage, there are still major difficulties involved with using solid-liquid phase change materials (PCM) as latent heat storage media. One is that PCM volume changes during phase transition. The volume change makes mechanical system design considerably difficult. Another difficulty is the maintenance of good heat conduction between solid-liquid PCMs and HTF. Heat transfer between the storage medium, HTS and working medium has not been solved properly, as the result, no commercial application of latent heat storage methods and apparatus have succeeded to date.